This invention relates to sizing agents for carbon fibers.
Carbon fiber reinforced plastics (hereinafter referred to as FRP) are made from carbon fiber with resin matrix and are one of the most excellent kinds of materials in terms of specific modulus and specific strength. Because of their superior characteristics and light weight, their applications in aerospace industries, as an example, are quickly expanding. Carbon fibers used for the production of FRP are drawn and arranged in the form of a filament or a tow and after they are formed into strands, sheets, textile, or knit materials, they are combined with a resin material and used as a prepreg. Alternatively, filaments or tows may be cut to uniform lengths to produce chopped fibers and after they are combined with a resin material, they may be used as a material for premix, a bulk molding compound or a sheet molding compound. Since carbon fibers are basically brittle, there tend to arise problems of fluffs due to mechanical friction in the molding process before they are made into a prepreg if they are not pretreated with a sizing agent. Moreover, they cannot be handled easily and the physical characteristics of the FRP are also adversely affected. As for chopped fibers, their lengths are usually 1-100 mm but since thousands or tens of thousands of single fibers constitute the carbon fiber filaments or tows which are processed, the fibers become disheveled and bulky and tend to scatter around easily if they are directly chopped without preprocessing. In order to improve the cohesiveness and abrasion resistance of carbon fibers and to make it easier to handle them while they are manufactured or transported, therefore, it has been a common practice to add a sizing agent to carbon fibers but since sizing agents eventually become a part of the produced FRP, it is required that they do not adversely affect the characteristics of the final products.
In view of such requirements as described above, the present invention relates to multi-purpose sizing agents to be combined with a matrix resin and in particular with epoxy resins and unsaturated resins having an ester bond.
As sizing agents for carbon fibers to be used for carbon fiber reinforced epoxy resin, a mixture of liquid and solid bisphenol A diglycidylethers (Japanese Patent Publication Tokko 57-15229, U.S. Pat. No. 3,914,504) and glycidylamines (Japanese Patent Publication Tokko 59-11710, U.S. Pat. No. 4,107,128) have previously been proposed. As sizing agents for carbon fibers to be used for carbon fiber reinforced unsaturated polyester resin, on the other hand, epoxidized polybutadiene (Japanese Patent Publication Tokkai 56-43335) and a mixture of a diglycidylether derived from bisphenol and epichlorohydrin and a prepolymer derived from diallylphthalate (Japanese Patent Publication Tokkai 59-228083) have been proposed. As still another example, sizing agents of the aqueous emulsion type having as indispensable components an epoxy resin, a condensation product of an unsaturated dibasic acid and a bisphenol-type alkylene oxide adduct, and an alkylene oxide adduct of phenol or polycyclic phenol have also been proposed (Japanese Patent Publication Tokko 57-49675, U.S. Pat. No. 4,167,538).
Although these prior art sizing agents each have certain advantages, they have problems regarding the production of FRP and their physical characteristics. For example, since carbon fibers have poor cohesiveness and abrasion resistance, problems of fluff and yarn breakage occur frequently at the time of their weaving and chopped fibers become disheveled. Some are toxic and flammable when exposed to high temperature because of the use of an organic solvent. Some may improve the adhesion between carbon fibers and epoxy resin matrix but not between carbon fibers and unsaturated polyester resin matrix such that interlaminar shear strength (ILSS) of the produced FRP is adversely affected. Sometimes, the attempt at improving the multi-purpose characteristics has resulted in insufficient adhesion characteristics regarding both types of resins.